The present invention relates to a device for selecting reception signals as a function of their power, such signals being identified by their frequency.
In the framework of the Pan European Digital Cellular Radiocommunications System known as the GSM system, terminals communicate by radio links with units connected to the public telephone network, these units being referred to as base stations. This system provides for 124 carriers of different frequency by means of which a terminal may send, and also provides for 124 carriers of different frequency again that a terminal can receive. Each carrier has a time-division structure and is formed by a periodic succession of frames each carrying eight equal time slots. Among a terminal's reception signals that are transferred by the carriers it is receiving, one particular signal, the broadcast signal, carries all the information allowing this terminal to enter into communication with the base station that is sending it.
The terminal needs to be able to identify the signal it is receiving with the highest strength since this most probably is the broadcast signal being sent by the base station to which it should connect itself, as will be explained below.
Moreover, the GSM system, by virtue of its operating mode, requires a terminal to store the identity and power of the strongest reception signals. As certain terminals are mobile, it is also established that the value of the power of a reception signal should be an average result from several measurements for example 8 spaced over time by a certain period, 10 milliseconds for example, thus making it possible to weight power variations at reception that are due to propagation conditions. In effect, a reception signal is the sum of several signals that have followed different paths, these latter signals, depending on the position of the mobile terminal, making a positive or a negative contribution to the received power.
One solution that has been suggested by the GSM system specifications consists in providing analysis means for selecting, over a measurement period, one time slot from each reception signal, this being done at the rate of appearance of the frame. This operation is repeated without interruption until the desired number of measurements has been obtained. Following the analysis means, power measuring means are provided which produce the values for the power of a signal as the average of the measurements made on the various time slots selected. The identity and the power of the selected signals are supplied by classification means which can access the various values that are established by the power measuring means.
This solution requires a large amount of time to identify the broadcast signal that is of interest, in other words the one for setting up the call, which is not desirable notably from the terminal user's point of view. Furthermore, the various measurements are made on time slots which occupy the same position in each one of the successive frames and this is not desirable for good discrimination of the broadcast signal. The base stations in effect permanently transmit this broadcast signal at maximum power, while in the majority of cases they transmit the other signals at a power that varies depending on the time slot, this power being notably adjusted as a function of the distance from the terminal for which such a signal is intended, and which always receives the same time slot. Thus, the average power calculated over several distinct time slots for a signal which is not the broadcast signal of the base station has a higher probability of being lower than the power of the broadcast signal from this base station than if average power was calculated from time slots occupying the same position.
The object of the present invention is hence to provide a device for selecting reception signals as a function of their power in which selection time is reduced and in which power measurements on a signal are carried out in distinct time slots.
The invention further provides supplementary means for further reducing this selection time when the powers of the various reception signals show great spread or dispersion.
The invention applies whatever type of selection is envisaged, whether identifying those signals that are received best for a determined number thereof is involved, or if this is done with respect to a determined threshold.
The invention is discussed in relation with the Pan European Digital Cellular Radiocommunications system in order to clearly present the inventive concept on the basis of a concrete case. The invention is not however limited to this field of application but applies each time that signals are being selected as a function of their power whenever such signals are structured into frames composed of time slots.